Report: Earth hit by severe solar storm much stronger than expected

The most powerful solar storm in nearly six years slammed Earth yesterday, but strangely, space weather forecasters didn't see it coming.

The geomagnetic storm peaked as a severe G4 on the 5-grade scale used by the U.S. National Oceanic and Atmospheric Administration (NOAA) to assess the severity of space weather events.

The storm's unexpected ferocity not only made auroras visible as far south as New Mexico in the U.S., but it also forced spaceflight company Rocket Lab to delay a launch by 90 minutes.

Geomagnetic storms are disturbances to Earth's magnetic field caused by solar material from coronal mass ejections (CME) large expulsions of plasma and magnetic field from the sun's atmosphere. 

It turns out that this particular geomagnetic storm was triggered by a "stealth" CME which as the name suggests is rather tricky to detect. 

NOAA's National Space Weather Service originally announced a "geomagnetic storm watch" on March 22 (opens in new tab), to come into effect on 23-25 March with possible moderate G2 storm conditions expected on March 24.

So forecasters weren't completely caught off-guard, they however didn't expect a magnitude G4 storm. 

U.S. space weather forecaster Tamitha Skov explained to Space.com in an email why the space weather community got it so wrong with this latest storm.